During the last two decades a number of so called “moisture adaptive vapour barriers” have been developed and later used in the building industry to a great extent. The overall idea behind the development of these vapour barriers (foils) has been to produce a vapour control layer which has a high vapour permeability in a moist environment and a low permeability in a dry environment. The desired effect has often been achieved in a combination of properties related to diffusion and capillary effect or diffusion alone. In this paper a new type of moisture adaptive vapour barrier based on diffusion will be presented. The Vapour control layer differs from earlier types, by having different properties dependent on the direction of the diffusion. This has been achieved by laminating materials with different properties and geometry (eg. Perforations) Results dealing with vapour permeability will be presented from standard cup tests and from a full scale test on a roof component.